High resistivity refractory with a high zirconia content

ABSTRACT

A novel fused and cast refractory product with a high zirconia content having improved electrical resistivity includes, as a percentage by weight relative to the oxides and for a total of more than 98.5%:
         ZrO 2 +Hf 2 O: &gt;85%   SiO 2 : 2% to 10%   Al 2 O 3 : 0.1% to 2.4%, with Al 2 O 3 /SiO 2 &lt;0.5   Y 2 O 3 : ≦1%,   B 2 O 3 : &lt;1.5%; and   a dopant selected from the group formed by V 2 O 5 , CrO 3 , Nb 2 O 5 , MoO 3 , Ta 2 O 5 , WO 3 , and mixtures thereof, in a weighted quantity such that:   0.2%≦2.43V 2 O 5 +8.84CrO 3 +1.66Nb 2 O 5 +6.14MoO 3 +Ta 2 O 5 +3.81WO 3 .

The invention relates to a novel fused and cast refractory product witha high zirconia content.

Refractory products include fused and cast products which are well knownfor the construction of glass fusion furnaces and sintered products.

Fused and cast products, as opposed to sintered products, usuallyinclude an intergranular vitreous phase connecting crystalline grains.Thus, problems with sintered products and fused and cast products andthe technical solutions adopted to overcome them are generallydifferent. A priori, then, a composition which has been developed for asintered product cannot per se be used for a fused and cast product, andvice versa.

Fused and cast products, frequently termed electrofused products, areobtained by fusing a mixture of appropriate starting materials in anelectric arc furnace or by any other technique which is suitable forsuch products. The molten liquid is then cast into a mold and theproduct obtained undergoes a controlled cooling cycle in order to bebrought to ambient temperature without fracturing. That operation istermed an “annealing” in the art.

Fused and cast products include electrofused products with a highzirconia content, i.e. comprising more than 85% by weight of zirconia(ZrO₂), which are known for their quality of having very high corrosionresistance without discoloring the glass produced and without generatingflaws.

Conventionally, fused and cast products with a high zirconia contentalso include sodium oxide (Na₂O) to prevent the formation of zircon fromthe zirconia and silica present in the product. Zircon formation isundesirable, as it is accompanied by a volume reduction of the order of20%, thereby creating mechanical stresses which are the source ofcracks.

Product ER-1195, which is produced and sold by the Société Buropéennedes Produits Réfractaires and disclosed in EP-B 403 387, is currentlywidely used in glass fusion furnaces. Its chemical composition comprisesabout 94% zirconia, 4% to 5% silica, about 1% alumina, 0.3% sodium oxideand less than 0.05% by weight of P₂O₅. It is typical of high zirconiacontent products used in glass furnaces.

FR2 701 022 describes fused and cast products with a high zirconiacontent which contain 0.05% to 1.0% by weight of P₂O₅ and 0.05% to 1.0%by weight of boron oxide B₂O₃ Such products have high electricalresistivity. They can advantageously stabilize electricity consumptionduring electric glass fusion and in particular can avoid any problemswith short circuiting in refractories, causing their rapid degradation.During electric glass fusion, part of the electric current passesthrough the refractories, An increase in the resistivity of suchrefractory products can thus reduce the quantity of electric currentwhich can pass through it.

WO-2005 068393 describes fused and cast products with a high zirconiacontent having high electrical resistivity, while minimizing thequantities of BaO, SrO, MgO, CaO, P₂O₅, Na₂O, and K₂O. Those productscontain 0.1% to 1.2% by weight of B₂O₃.

Present developments towards very high quality glass, in particularglass for LCD type flat screens, increases demands for refractoryproducts from glass fusion furnaces. In particular, there is a need forrefractory products having further improved electrical resistivity whileretaining good resistance to corrosion by the molten glass.

The present invention seeks to satisfy that need.

More particularly, it provides a fused and cast refractory product witha high zirconia content comprising, as percentages by weight relative tothe oxides and for a total of more than 98.5%, preferably more than 99%and more preferably more than 99.5%:

-   -   ZrO₂+Hf₂O: >85%    -   SiO₂: 1% to 10%    -   Al₂O₃: 0.1% to 2.4%    -   B₂O₃: <1.5%; and    -   a dopant selected from the group formed by V₂O₅, CrO₃, Nb₂O₅,        MoO₃, Ta₂O₅, WO₃ and mixtures thereof, in a weighted quantity        expressed by the following formula (1):

0.2%≦2.43V₂O₅+8.84CrO₃+1.66Nb₂O₅+6.14MoO₃+Ta₂O₅+3.81WO₃

and preferably by the following formula (2):

0.2%≦2.43V₂O₅+4.42CrO₃+1.66Nb₂O₅+3.07MoO₃+Ta₂O₅+1.91WO₃

As shown below, surprisingly, the refractory product of the inventionhas a remarkable electrical resistivity while retaining good resistanceto corrosion by the fused glass.

Preferably, the refractory product of the invention also exhibits oneor, as is preferable, several of the following optional characteristics:

-   -   the weighted quantity of dopant is 0.5% or more, preferably 0.6%        or more, more preferably 1.2% or more and/or 3% or less,        preferably 2.5% or less, more preferably 1.4% or less;    -   the dopant is selected from V₂O₅, Nb₂O₅, Ta₂O₅, WO₃, and        mixtures thereof, preferably from Nb₂O₅, Ta₂O₅, and mixtures        thereof;    -   the quantity of silica, SiC₂, is 2% or more, preferably 3% or        more, preferably 3.8% or more, and/or 8% or less;    -   the Al₂O₃/SiO₂ ratio is less than 0.5, preferably less than 0.3,        and more Preferably less than 0.25. This characteristic is        particularly advantageous when the silica content is less than        2%.    -   the quantity of B₂O₃ is more than 0.05%, preferably more than        0.1%, and/or less than 1%. A quantity of B₂O₃ more than 0.1%, or        more than 0.2%, or even more than 0.25%, is desirable, in        particular, when SiO₂<3%.    -   the product does not contain V₂O₅.    -   the quantity of yttrium oxide, Y₂O₃, is 1% or less, preferably        less than 0.5%, more preferably less than 0.2%;    -   the quantity of boron oxide, B₂O₃, is 1% or less, preferably        less than 0.50%;    -   the quantity of zirconia, ZrO₂+Hf₂O, is 90% or more, preferably        93% or more;    -   the quantity of alumina, Al₂O₃, is 0.5% or more, preferably 0.6%        or more, and/or 1.5% or less, preferably 1% or less, even more        preferably 0.85% or less;    -   the refractory product has the following composition, with        ZrO₂+HfO₂ as the complement to 100%:        -   SiO₂: 3.8% to 4.8%        -   B₂O₃: <0.25%        -   Al₂O₃: 0.65% to 0.85%        -   Y₂O₃: <0.45%        -   between 0.8% and 1.2% of Ta₂O₅ or between 0.4% and 0.9% of            Nb₂O₅.    -   the quantity of impurities (mainly iron, titanium, phosphorous,        and calcium oxides) is less than 0.6%, preferably less than        0.3%.

Advantageously, these characteristics can further improve the electricalresistivity and corrosion resistance of the product of the invention.

The refractory product of the invention preferably has electricalresistivity of 200 Ω.cm [ohm.centimeter] or more, preferably 400 Ω.cm ormore, more preferably 600 Ω.cm or more at 1500° C. and at a frequency of100 Hz [hertz]. At 950° C., this resistivity may be 1000 Ω.cm or more,preferably 20000 Ω.cm or more, and more preferably 25000 Ω.cm or more.It may even be 28000 Ω.cm or more.

The invention also provides a glass fusion furnace including arefractory product of the invention, or a refractory product that isfabricated or capable of being fabricated using a method of theinvention, in particular in regions intended to come into contact withthe molten glass. In a furnace of the invention, the refractory productmay advantageously form part of a cell for preparing the glass to befused, in particular by electrical fusion, where it may come intocontact with molten glass at a temperature of more than 1200° C.

The refractory product of the invention is not intended to come intocontact with molten glass at temperatures below 1100° C.

The invention also provides an electrolysis cell, for example for theelectrolysis of aluminum, comprising a plurality of refractory blocks,at least one of said blocks being a refractory product of the inventionor a refractory product fabricated or capable of being fabricated usinga method of the invention. In particular, this product may form part ofthe side wall of the cell. It may be disposed in a region where it maycome into contact with molten cryolite.

Finally, the invention provides a process for fabricating a refractoryproduct of the invention, the process comprising the following steps insuccession:

-   -   a) mixing the starting materials, including introducing a        dopant, to form a starting charge;    -   b) fusing said starting charge to obtain a molten liquid; and    -   c) casting and solidifying said molten liquid by controlled        cooling to obtain a refractory product; said process being        remarkable in that said starting materials are selected so that        the refractory product is in accordance with the invention.

The amount or “weighted” quantity of dopant as used here designates thequantity:

2.43V₂O₅+8.84CrO₃+1.66Nb₂O₅+6.14MoO₃+Ta₂O₅+3.81WO₃

and preferably the quantity:

2.43V₂O₅+4.42CrO₃+1.66Nb₂O₅+3.07MoO₃+Ta₂O₅+1.91WO₃

where the amounts of the oxides are expressed as percentages by weight.

Unless otherwise stated, all of the percentages in the presentdescription are percentages by weight relative to the oxides.

In the fused and cast products of the invention, the high zirconiacontent, i.e. ZrO₂>85%, enables the demand for high corrosion resistanceboth without discoloring the glass produced and not generating defectswhich may harm the quality of that glass to be satisfied.

Hafnium oxide, HfO₂, present in the product of the invention is thehafnium oxide which is naturally present in sources of zirconia. Itscontent in the product of the invention is thus 5% or less, generally 2%or less.

The presence of silica is necessary for the formation of anintergranular vitreous phase which can effectively accommodatevariations in the volume of the zirconia during its reversibleallotropic transformation, i.e. during the change from the monoclinicphase to the tetragonal phase. In contrast, the added silica must notexceed 10% since, because it is added to the detriment of the zirconiacontent, corrosion resistance would be reduced.

The presence of alumina is necessary for the formation of a stablevitreous phase and good flow characteristics of the products in themold. An excessive amount would cause the vitreous phase to becomeunstable (crystal Formation).

Preferably, the product of the invention comprises a quantity of B₂O₃which is 0.5% or less. Boron oxide actually has an unfavorable effect onthe formation of zircon in the product. That element can improve theproduct feasibility.

Yttrium oxide, Y₂O₃, has an unfavorable effect on electricalresistivity, but its presence has to be tolerated in a quantity of lessthan 1%, preferably less than 0.5%, more preferably less than 0.2%.

The dopant has to be present in the products of the invention to improvethe electrical resistivity. However, the total weighted content of saidoxides preferably must not exceed 4% so that the percentage of zirconiais maintained at a level which is sufficiently high to ensure excellentresistance to corrosion by the molten glass and to keep the vitreousphase stable.

The inventors have established that all pentavalent dopants have asubstantially identical effect in identical molar quantities. This isthe same for all hexavalent dopants. Further, the inventors haveobserved a molar efficiency for hexavalent dopants M⁶⁺ which is aboutdouble that for pentavalent dopants M⁵⁺. Without wishing to be bound bya particular theory, the inventors explain this difference by the rolethe dopants play as regards oxygen voids in the zirconia. The hexavalentdopants M⁶⁺ would in fact compensate for two oxygen voids, as opposed toa single void for the pentavalent dopants M⁵⁺. One mole of an oxide of apentavalent dopant M₂O₅ would thus have an identical effect to one moleof an oxide of a hexavalent dopant MO₃.

The weighted dopant content also takes into account differences betweenthe molar masses of the dopants. Thus, 1.66 grams of Ta₂O₅ have aneffect equivalent to one gram of Nb₂O₅.

The complement to 100% in the composition of the product of theinvention is constituted by the other species. The term “other species”means species the presence of which is not particularly desirable andwhich are generally present in the starting materials as impurities.

Alkali oxides may be mentioned, in particular sodium oxide Na₂O andpotassium oxide K₂O, which may be tolerated but preferably must notexceed 0.5%, preferably 0.1%, and more preferably only be present intrace quantities. Otherwise, the electrical resistivity would bedegraded due to the increased conductivity of the vitreous phase. Oxidesof iron, titanium and phosphorus are known to be deleterious and theiramount must be limited to traces introduced with the starting materialsas impurities. Preferably, the quantity of Fe₂O₃+TiO₂ is less than 0.55%and that of P₂O₅ is less than 0.05%.

A product of the invention may be fabricated in steps a) to c) describedbelow:

-   -   a) mixing the starting materials, including introducing a dopant        to form a starting charge;    -   b) fusing said starting charge to obtain a molten liquid; and    -   c) solidifying said molten liquid by controlled cooling to        obtain a refractory product in accordance with the invention.

In step a), the dopant is added to guarantee an amount of dopant in thefinished product of the invention.

In step b), fusion is preferably carried out by the combined action of arelatively long electric arc, producing no reduction, and agitation toencourage re-oxidation of the products.

To minimize the formation of nodules with a metallic appearance andavoid the formation of cracks or crazing in the final product, it ispreferable to carry out fusion under oxidizing conditions.

Preferably, the long arc fusion process described in French patent1208577 and its patents of addition, numbers 75893 and 82310, is used.

That process consists of using an electric arc furnace the arc of whicharcs between the charge and at least one electrode distanced from saidcharge; the length of said arc is adjusted so that its reducing actionis reduced to a minimum while maintaining an oxidizing atmosphere abovethe fusion melt and agitating said melt either by the action of the arcitself or by blowing an oxidizing gas (air or oxygen, for example) intothe melt, or by adding oxygen-releasing substances such as peroxides tothe melt.

In step c), cooling is preferably carried out at a rate lower than 20°C. per hour, preferably at a rate of about 10° C. per hour.

Any conventional process for fabricating fused products based onzirconia intended for applications in glass fusion furnaces may becarried out, provided that the composition of the starting charge canproduce products with a composition in accordance with that of theproduct of the invention.

The following non-limiting examples are given by way of illustration ofthe invention.

In the examples, the following starting materials were used:

-   -   zirconia principally containing, on average by weight, 98.5% of        ZrO₂+HfO₂, 0.2% of SiO₂ and 0.02% of Na₂O;    -   zircon sand containing 33% silica;    -   AC44 type alumina sold by Pechiney and containing an average of        99.4% of alumina, Al₂O₃; and    -   oxides of boron, yttrium, tantalum Ta₂O₅ and niobium Nb₂O₅ with        a purity of more than 99%.

The products 1 to 39 were prepared using a conventional arc furnacefusion process then cast to obtain blocks with dimensions of 220×450×150mm millimeter.

The products 40 to 43 were manufactured using a method of fusion byinduction, as described in document FR 1 430 962, with a 275 mm diameterturn, power lying in the range 120 kW to 220 kW, and a frequencydelivered by the aperiodic generator lying in the range 100 kHz to 250kHz.

The chemical analysis of the products obtained is given in Table 1: thismean chemical analysis is given as a percentage by weight.

In this table, an empty cell corresponds to a quantity of 0.05% byweight or less. (*) indicates that the example is outside the scope ofthe invention.

For the various examples of blocks which were produced, cylindrical bars30 mm in diameter and 30 mm high of the product were subjected to apotential difference of 1 volt at a frequency of 100 Hertz at 1500° C.to carry out the measurements of electrical resistivity, R.

TABLE 1 Ta₂O₅ + 1.66 R ZrO₂ SiO₂ B₂O₃ Al₂O₃ Al₂O₃/SiO₂ Na₂O Nb₂O₅ Ta₂O₅Y₂O₃ Nb₂O₅ (Ω · cm)  1* 94.5 4.0 1.2 0.30 0.3 70  2* 91.3 7.0 0.6 1.10.16 95  3* 94.6 4.3 0.3 0.5 0.12 0.1 0.2 139  4* 89.8 8.3 0.6 1.2 0.140.1 165  5 93.0 5.4 0.6 0.5 0.09 0.2 0.3 0.2 221  6 94.0 4.4 0.3 0.90.20 0.2 0.2 0.2 217  7 92.8 5.5 0.4 0.9 0.16 0.3 0.1 0.5 335  8 94.14.0 0.3 0.9 0.23 0.5 0.2 0.5 334  9 92.9 5.1 0.5 0.8 0.16 0.6 0.1 0.6376 10 94.8 3.2 0.2 0.7 0.22 0.6 0.5 0.6 249 11 94.1 4.0 0.1 0.7 0.180.7 0.4 0.7 680 12 93.8 4.1 0.5 0.8 0.20 0.7 0.1 0.7 390  13* 93.3 3.60.3 0.7 0.19 0.7 1.4 0.7 63 14 94.5 3.9 0.3 0.8 0.21 0.4 0.1 0.7 325 1593.3 4.6 0.4 0.7 0.15 0.1 0.8 0.1 0.8 244 16 93.5 4.4 0.4 0.8 0.18 0.80.1 0.8 370 17 91.0 6.1 0.5 1.0 0.16 0.1 0.8 0.5 1.0 273 18 94.1 4.0 0.30.9 0.22 0.6 0.1 1.0 558  19* 96.3 1.5 0.2 0.9 0.57 1.0 0.2 1.0 95 2093.6 4.1 0.3 0.9 0.21 1.1 0.1 1.1 526 21 90.4 6.6 0.7 0.9 0.14 0.1 0.30.6 0.4 1.1 346 22 93.2 4.4 0.3 0.9 0.19 1.2 0.1 1.2 528 23 93.5 4.6 0.20.8 0.18 0.8 0.1 1.3 648 24 94.4 3.7 0.2 0.9 0.23 0.8 1.3 404 25 93.34.3 0.9 0.20 1.4 0.1 1.4 436 26 93.3 3.5 0.3 0.7 0.20 1.4 0.8 1.4 204 2793.8 4.1 0.8 0.20 0.3 0.9 0.1 1.4 359 28 88.1 8.4 0.6 1.2 0.15 1.6 0.11.6 456 29 93.0 4.6 0.3 0.9 0.20 0.6 0.6 1.6 477 30 92.2 5.0 0.4 0.70.14 0.3 1.2 0.2 1.7 373 31 89.4 6.8 0.5 1.4 0.20 1.7 0.2 1.7 448 3293.4 4.0 0.3 0.8 0.20 0.8 0.5 0.2 1.8 427 33 92.3 4.5 1.0 0.22 2.1 0.12.1 335 34 90.9 5.2 0.4 0.7 0.13 0.2 2.0 0.6 2.3 360 35 92.5 4.4 0.4 0.70.16 1.5 0.5 2.5 261 36 91.5 4.7 0.4 0.8 0.16 0.3 2.1 0.2 2.6 253 3796.9 1.5 0.3 0.5 0.33 0.7 0.1 1.2 356 38 90.2 7.4 0.9 0.3 0.04 1.2 1.2322 39 86.5 8.8 1.4 1.5 0.17 1.8 1.8 480 40 94.0 3.7 0.3 0.8 0.22 1.21.2 448 41 94.9 3.5 0.3 0.8 0.23 0.5 0.8 460 42 94.6 3.6 0.2 0.8 0.220.8 1.3 429 43 93.3 4.2 0.3 1.1 0.26 0.5 0.6 1.4 370

These examples show that adding dopants can significantly increase theelectrical resistivity of fused and cast refractory products with a highzirconia content when the total weighted dopant content(Ta₂O₅+1.66Nb₂O₅) is more than 0.2%, preferably more than 0.5% as apercentage by weight relative to the oxides. The products of theinvention thus have an electrical resistivity of more than 200 ohm.cm.

The examples of Table 1 also show the molar equivalence of the variousdopants. This equivalence is especially clear if the values(Ta₂O₅+1.66Nb₂O₅) and the resistivity R of Examples 7 and 8 or 12 and 14are compared.

Other tests have shown that when the total weighted dopant content isincreased beyond 3% by weight, no additional increase in electricalresistivity is obtained. It is more preferable to limit the totalweighted dopant content to 3% by weight so that the percentage ofzirconia is maintained at a sufficiently high level to ensure excellentresistance to corrosion by the molten glass.

The examples, in particular example 23 which corresponds to thepreferred composition, also show that the effect of adding dopant is amaximum when the total weighted dopant content is in the range 0.5% to2% as a percentage by weight.

Example 19* and a comparison of Examples 9 and 10 or 23 and 24 indicatethat increasing the silica content is favorable for improving theelectrical resistivity of the products. The silica content must thus begreater than 2%, preferably greater than 3% as a percentage by weightrelative to the oxides.

It should also be noted that it is advantageous for the products topresent a quantity of B₂O₃ that is more than 0.05%, preferably more than0.1%, or even more than 0.2%, in particular when SiO₂<3%.

Example 13*, a comparison of Examples 23 and 24 and to a lesser extentExample 26 indicate that an increase in the yttrium oxide content isdeleterious if the electrical resistivity of the products is to beimproved.

It should also be noted that it is preferable to limit the Na₂O contentto values of 0.1% or less, preferably 0.05% or less. Preferably, theproduct of the invention includes only traces of Na₂O. As can be seen bya comparison of Examples 15 and 16, the presence of sodium oxide Na₂Ohas a deleterious effect on the electrical resistivity due to the lowresistivity of the vitreous phase.

Finally, it can be observed from Examples 40 to 43 that using aninduction furnace also results in good electrical resistivity. Othertests have also shown that, surprisingly, the use of an inductionfurnace with continuous fusion and solidification, as described in FR 1430 962, can produce products with a particularly homogeneous zirconiacontent.

Further, other tests have shown that the other recognized properties ofhigh zirconia content materials, in particular resistance to corrosionby glass, are not degraded by the presence of a dopant in accordancewith the invention.

The products of the invention may advantageously be used in any otherapplication requiring a refractory product having a high electricalresistivity. In particular, such products may be used to constructaluminum electrolysis cells in which aluminum metal can be produced onan industrial scale by electrolysis of alumina in solution in acryolite-based melt.

The electrolyte is conventionally contained in an electrolysis cell. Thecell comprises a side wall and a base. The base is composed ofrefractory base blocks and cathode blocks with insulating blocks at thelower portion. The side wall is formed from refractory side blockssurrounded by a metal envelope or casing which is insulated to a greateror lesser extent.

The blocks are used at temperatures of 950° C. or less.

The electrical resistivity at 950° C. of Example 18 of the invention wasthus compared with that of a reference block based on silicon carbide(SiC) bonded by a silicon nitride (Si₃N₄) matrix using the same protocolas above, but at a temperature of 950° C.

The electrical resistivity at 950° C. of Example 18 was 30000 Ω.cm,while that of the reference block was 6000 Ω.cm.

The resistance to corrosion by cryolite was evaluated by keeping sampleswith a cross section of 25 mm×25 mm of Example 18 and the referenceblock in a cryolite melt for 22 hours at 1030° C.

The sample of Example 18 had a corroded volume (reduction in volume dueto corrosion) which was half that of the reference block.

The refractory products of the invention are thus entirely suitable foruse in an electrolysis cell, in particular for aluminum, moreparticularly as an element of a side wall of said cell and/or in a zonewhere they can come into contact with molten cryolite.

Clearly, the present invention is not limited to the embodimentsdescribed and shown by way of non-limiting illustrative examples.

1-29. (canceled)
 30. A fused and cast refractory product with a highzirconia content comprising, as percentages by weight relative to theoxides and for a total of more than 98.5%: ZrO₂+Hf₂O: >85% SiO₂: 1% to10% Al₂O₃: 0.1% to 2.4%, B₂O₃: <1.5%; and a dopant selected from thegroup formed by CrO₃, Nb₂O₅, MoO₃, Ta₂O₅, WO₃, and mixtures thereof, ina weighted quantity such that:0.2%≦8.84CrO₃+1.66Nb₂O₅+6.14MoO₃+Ta₂O₅+3.81WO₃.
 31. A refractory productaccording to claim 30, in which said weighted quantity is such that:0.2%≦4.42CrO₃+1.66Nb₂O₅+3.07MoO₃+Ta₂O₅+1.91WO₃.
 32. A refractory productaccording to claim 30, in which the quantity of silica SiO₂ is 2% ormore.
 33. A refractory product according to claim 30, in which theAl₂O₃/SiO₂ ratio is less than 0.5.
 34. A refractory product according toclaim 33, in which the Al₂O₃/SiO₂ is less than 0.3.
 35. A refractoryproduct according to claim 30, in which the weighted quantity of dopantis 0.5% or more, and 3% or less.
 36. A refractory product according toclaim 30, in which the weighted quantity of dopant is 0.6% or more and1.4% or less.
 37. A refractory product according to claim 30, in whichthe dopant is selected from Nb₂O₅, Ta₂O₅, WO₃ and mixtures thereof. 38.A refractory product according to claim 30, in which the dopant isselected from Nb₂O₅, Ta₂O₅ and mixtures thereof.
 39. A refractoryproduct according to claim 30, in which the quantity of silica, SiO₂, is3% or more and 8% or less.
 40. A refractory product according to claim30, in which the quantity of B₂O₃ is more than 0.05% and less than 1%.41. A refractory product according to claim 40, in which the quantity ofB₂O₃ is more than 0.1%.
 42. A refractory product according to claim 40,in which the quantity of SiO₂ is less than 3%.
 43. A refractory productaccording to claim 30, in which the quantity of Y₂O₃ is 1% or less. 44.A refractory product according to claim 30, having an electricalresistivity of at least 200 Ω.cm at 1500° C. and/or at least 10000 Ω.cmat 950° C.
 45. A refractory product according to claim 30, having anelectrical resistivity of at least 400 Ω.cm at 1500° C. and/or at least25000 Ω.cm at 950° C.
 46. A refractory product according to claim 30,having an alumina (Al₂O₃) content of 1% or less, as a percentage byweight based on the oxides.
 47. A refractory product according to claim46, having an alumina (Al₂O₃) content of 0.85% or less, as a percentageby weight based on the oxides.
 48. A refractory product according toclaim 30, having a Na₂O content of less than 0.1%.
 49. A refractoryproduct according to claim 30, having the following composition, whereinZrO₂+HfO₂ and the impurities form the complement to 100%: SiO₂: 3.8% to4.8%; B₂O₃: <0.25%; Al₂O₃: 0.65% to 0.85%; Y₂O₃: ≦0.45%; 0.8% to 1.2% ofTa₂O₅ or 0.4% to 0.9% of Nb₂O₅.
 50. A process for fabricating arefractory product, comprising the following steps: a) mixing startingmaterials to form a starting charge, b) fusing said starting charge toobtain a molten liquid, c) casting and solidifying said molten liquid bycontrolled cooling to obtain a refractory product, said process beingcharacterized in that said starting materials are selected so that therefractory product is in accordance with claim
 30. 51. A processaccording to claim 50, wherein fusion is carried out in oxidizingcondition.
 52. A process according to claim 50, wherein fusion iscarried out with an induction furnace or with a long arc.
 53. A processaccording to claim 50, wherein, at step c), cooling is carried out at arate lower than 20° C. per hour.
 54. A glass fusion furnace,characterized in that it includes a refractory product according toclaim
 30. 55. A furnace according to claim 54, said refractory productforming a part of a cell for preparing the glass by electrical fusion,where it may come into contact with molten glass at a temperature higherthan 1200° C.
 56. An electrolysis cell comprising a plurality ofrefractory blocks, characterized in that at least one of said blocks isa refractory product according to claim
 30. 57. A cell according toclaim 56, wherein said refractory product may come into contact withmolten cryolite.
 58. A cell according to claim 56, in which saidrefractory product forms part of the side wall of the cell.